Diazo printers have been known for many years, and are especially effective for producing high-accuracy whiteprints from original materials to be printed. In such printers, a sheet of printing paper and an overlying sheet of original material to be printed are conveyed by a motor-driven roller system through a position in which they are exposed to ultra-violet radiation from a suitable ultra-violet lamp, after which the original document and the copy paper are separated from each other; the original is returned to the machine user, while the copy paper is moved, again by the motor-driven roller system, through a developer chamber in which it is exposed to a mixture of ammonia and water vapor. Upon exit from the developing chamber, the copy paper carries an accurate reproduction of the information contained upon the original copy.
In such a system it is important to provide the proper proportion of water vapor mixed with ammonia gas in the developing chamber, or else proper image development may not occur. The water vapor is produced by an evaporator, supplied with water from a suitable water source or from aqua ammonia, and the evaporator is provided with a suitable thermostatically controlled heater operating in response to a temperature sensor within the evaporator, arranged so that, over a period of time, the amount of moisture evaporated into the developer chamber is substantially the same as the amount of water supplied to the evaporator from the water source. Accordingly, in order to maintain the desired water-vapor concentration in the developer chamber, it is desirable to supply water to the evaporator at the corresponding desired rate, and it has been found that the optimum amount of water vapor and thus water depends upon the type of reproducing paper used.
In the past it has also been recognized that it is desirable for many purposes to vary the rate of delivery of water to the evaporator as a function of the speed of the motor which moves the copy paper through the machine. Thus when the machine runs more rapidly, so that more footage of print paper per minute passes through the developer, a greater rate of delivery of water vapor and hence of water to the evaporator, is desired. In the past this has been accomplished by manual adjustment of a water supply valve, or automatically by means of an arrangement of multi-lobe cam switches mechanically coupled to the motor drive system, each lobe of any selected one of the cam switches serving to close an electrical circuit periodically and operate a solenoid pump to meter a predetermined quantity of water into the evaporator. Each multi-lobe cam switch produced an increasing rate of water supply to the evaporator as the motor speed increased, and by using different numbers of lobes on the different cams, one was able to select the absolute rate of flow of water into the evaporator for any motor speed.
While satisfactory for certain purposes, the latter types of arrangement have proved to be rather bulky, expensive, and less versatile than is desirable for many purposes.
Accordingly, it is an object of the invention to provide an improved diazo printer.
Another object is to provide an improved system for controlling the rate of delivery of water to the water evaporator as a function of the speed of the drive motor of a diazo printer.
Another object is to provide such a system in which the rate of water delivery at any given motor speed can be manually adjusted.
It is also an object to provide such an improved system which is of high accuracy, versatile, and relatively inexpensive.